(4/9):(2/3)=(1/2):(3/4) (4/9):(2/3)=(1/3)/(1/2)(2/3):(1/2)=(4/9):(1/3)加的数就是已知数没有的
1/2×2/3÷4/9=3/41/2×4/9÷2/3=1/32/3×4/9÷1/2=4/27
